The pontiff, 85, is the first to give up his post in 600 years. His eight-year tenure was rife with challenges.

ROME — During nearly eight years leading the world's 1 billion Roman Catholics, Pope Benedict XVI zealously pursued a vision of orthodox belief and conservative practice -- yet he upset centuries of tradition with his unexpected decision to resign.

His announcement Monday that he would step down at the end of the month was a stunning denouement virtually no one expected. Benedict, 85, said increasing infirmity made it difficult for him to dispatch his duties with energy and diligence, triggering his decision to become the first pontiff in nearly 600 years to give up his post.

He will retire to a Vatican building formerly occupied by nuns, presenting the church with the dilemma of how a current and former pope might coexist.

"After having repeatedly examined my conscience before God, I have come to the certainty that my strengths, due to an advanced age, are no longer suited to an adequate exercise of the Petrine ministry," the pope declared.

For The Record Los Angeles Times Tuesday, February 26, 2013 Home Edition Main News Part A Page 4 News Desk 1 inches; 39 words Type of Material: Correction Selecting a pontiff: An illustration in the Feb. 12 Section A that accompanied an article on the resignation of Pope Benedict XVI included an image of a chalice with a lid beside it. Traditionally, chalices do not have lids.

Benedict was confronted during his tenure with keeping the church relevant in an increasingly secular age, but also shepherding it through one of its gravest crises in modern history: the allegations of serial sexual abuse by clerics around the world.

In addition, Benedict contended with infighting among senior officials, accusations of corruption inside the Vatican and the leak of embarrassing private documents by his personal butler.

The strain of office was evident in recent months as the pope cut down on his public appearances. A quiet, gentle scholar who was never one to draw energy from crowds, Benedict looked even more frail and careworn than usual.

Although there have been no reports of specific illnesses, he has been walking with greater difficulty. He uses a wheeled platform to go to and from the altar in St. Peter's Basilica, and his brother told the German news agency DPA that a doctor had advised against any more transatlantic trips.

Even though he had once described resignation as an option for a pontiff who had become debilitated, his announcement flabbergasted veteran Vatican watchers.

"He was really tired, but I didn't think that he could be that tired," said Marco Tosatti, who writes for Italy's La Stampa newspaper.

"It's a really great sign of freedom of spirit, saying: 'OK, I'm not able anymore to do all that is necessary for the church. I have to accept it.' "

Vatican officials said a new pope would probably be installed by Easter, which falls on March 31.

Speculation immediately focused on the possibility that a successor could come from the developing world -- perhaps from Africa, where the church is flourishing.

Whoever it is will be likely to continue the German-born Benedict's emphasis on doctrinal conservatism and conformity.

The College of Cardinals, which will elect a new pontiff from among its ranks, is packed with proteges who share his belief that theological purity is the key to renewal, especially within Europe. Under Benedict, the Latin Mass came back in; innovations such as female priests or a tolerant attitude toward homosexuality remained firmly out.

"He put a lot of effort and care in choosing the bishops," Tosatti said. "He can leave more or less confident saying to himself, 'I've tried to put the best person in my judgment to do the job.' "

Vatican officials acknowledged the oddity of having two popes around at the same time.

John Thavis, an expert on the Vatican, said the possibility that Benedict's presence would undermine his successor shouldn't be exaggerated. "But there might be situations where anything he writes

Benedict was considered a safe pair of hands when, as Cardinal Joseph Ratzinger, he was selected in April 2005. The oldest man to become pope in nearly 300 years, he was regarded by many as a caretaker wedged between his charismatic predecessor, John Paul II, and a dynamic, future leader.

His outward persona -- stiff, bookish and ill at ease -- never endeared him to the public, though he is said to be engaging and humorous in private, a cultured man who plays the piano and loves cats and designer shoes.

But he was an ardent traditionalist, "God's Rottweiler" to some, and he set about shoring up conservative elements of Roman Catholicism. He drew on his 24 years of experience cracking down on dissent as head of the Congregation for the Doctrine of the Faith, John Paul II's chief enforcer of orthodoxy.

Within months of his elevation, Benedict issued a reaffirmation of the ban on men with "deep-seated homosexual tendencies" from the priesthood.

Yet his first encyclical, the weightiest document a pope can produce, centered on love and the tenet that God is love.

It was clear from the beginning that Benedict would focus on his own backyard. One of his primary goals was to re-evangelize Europe, which he believed had passed through the evils of Marxism, liberalism and materialism on a march to godlessness.